The morphological diversity among neurons is enormous and critical to their function, yet many gaps in our knowledge remain concerning how this diversity is genetically encoded. As with vertebrate motor neurons, the motor neurons of Drosophila melanogaster establish highly specific synapses on the correct muscle fibers in the appendages and also elaborate highly stereotyped dendritic arbors in the central nervous system (CNS). In the previous funding period, this diversity of motor neuron morphology was defined at the single cell level for the 47 motor neurons that innervate and control the movements of each of the adult legs of Drosophila. Further, sets of transcription factors were characterized that act in a combinatorial manner to dictate the morphologies of seven motor neurons, all derived from the same neuroblast stem cell. An additional set of factors was defined that are expressed in a temporally distinct manner in a stem cell lineage that gives rise to 28 motor neurons. Based on these findings, one goal for the next funding period is to determine how combinatorial TF codes are established in post-mitotic neurons, by testing the hypothesis that they are regulated by factors acting earlier in the neuroblast. A second goal is to identify te genes and pathways regulated by TFs in post-mitotic motor neurons that control their synaptic specificity in the adult legs and dendritic architecture in the CNS. A combination of genetic screens and molecular approaches will be used. A third long term goal is to determine how the adult leg muscles develop coordinately with motor neurons to establish synaptic specificity. Using a highly quantitative method for analyzing fly walking, the consequences on walking behavior by perturbing the development of these neurons and/or muscles will be analyzed. In general, these studies will link the development of a highly stereotyped set of neurons to their function in a specific adult behavior.